The present invention relates in general to a motor vehicle braking system and, more particularly, this invention relates to a system which will minimize an unintentional response of a respective brake solenoid control valve due to an axle oscillation, and which will prevent locking of a respective braked wheel when such respective braked wheel is subjected to a full brake application, even on a slippery highway surface.
In order to reduce the stopping distance of a motor vehicle, especially on a relatively slick road surface, it is known in the braking art that locking of the brakes on a respective wheel being subjected to a full brake application must be avoided. Furthermore, it is known that when such locking of the brakes is prevented, the operator of such vehicle can retain enhanced steering capabilities.
It is likewise generally recognized in the braking art that when the rotary deceleration (-b) of a respective braked wheel exceeds a certain threshold value, then the locking of such respective braked wheel will normally occur. The threshold value normally ranges above the value that is to be expected for the highest possible retardation of such vehicle without experiencing the undesirable locking of the respective braked wheels.
Prior to the present invention, a motor vehicle brake antilocking system had been developed and used. This known brake antilocking system reacts to a signal that is representative of a respective wheel rotary deceleration (-b) with a resulting reduction of the brake pressure within a brake cylinder of such respective braked wheel by activation of an associated solenoid control valve.
However, it has been determined in use that with this particular antilocking system, an undesirable control signal can be generated, which temporarily actuates the solenoid control valves. It has been found that this may occur even during normal travel conditions. For example, these undesirable control signals have been generated either with or without the brakes on such vehicle being applied. This false activation of such solenoid control valves results in their added wear and subsequent required repair and/or replacement which, in turn, adds to the maintenance cost for the vehicle owner.
Persons skilled in the braking art believe that one of the primary causes of such detrimental control signals is the occurrence of the so-called axle oscillation. As used in this specification, the term "axle oscillation" is understood to mean a torsional oscillation of the wheels with an approximate frequency of between about 8 Hertz and about 10 Hertz. Such torsional oscillation of the wheels can be caused, by way of example, from a relatively severe unevenness of the roadway surface. An axle oscillation of this type may also occur during a partial braking process. In either case, such axle oscillation will usually result in an undesirable loss of pressure for the brake cylinders due to the responding of the solenoid control valve to an undesirable control signal.
In order to eliminate this erratic performance of the electronic antilocking system, it was believed possible that a manometric switch could be installed within the braking circuitry of such vehicle. In this type of arrangement, the electronic antilocking system would be actuated by the manometric switch when and only when a particular preselected pressure was present. This task is accomplished, in this case, by switching back to a reduced value a previously, constantly increased threshold value for the rotary deceleration (-b) upon reaching such preselected pressure. However, even in this arrangement, it is still possible that,in the event of a partial brake application on such vehicle, there may occur unnecessary solenoid control valve actuations in the antilocking system. Of even greater concern is the possible danger that could be encountered if a manometric switch developed a fault, which could cause the antilocking system to respond to a control signal too late. Furthermore, this arrangement has the added disadvantage of significantly increasing the cost associated with such antilocking system, not only because of the cost of the manometric switch, but also because of the necessary extra electrical connections required to install such manometric switch to the electronic circuitry of such antilocking system.
Further known in the braking art from German Patent No. 2,726,738, the teachings of which are incorporated herein by reference thereto, is to detect an axle oscillation through generation of a (+b) signal which is representative of wheel-turning acceleration, and to then raise the rotary deceleration (-b) threshold. Although this arrangement will usually prevent the generation of an erroneous control signal, the possibility still exists with this system, that due to the increased (-b) threshold, a subsequent controlled braking process will not proceed at an optimum level.